1. Field of the Invention
The invention relates to an apparatus for casting metal strip. More particularly, the invention relates to a nozzle assembly system for mounting a casting nozzle to a crucible reservoir to cast continuous metal strip.
2. Description of the Prior Art
In the production of continuous metal strip, molten metal has typically been extruded from a pressurized reservoir through a nozzle on to a high speed, rotating quench surface. Representative apparatus are shown in U.S. Pat. No. 4,142,571 for "continuous casting method for metallic strips" issued Mar. 6, 1978 to M. Narasimhan.
When continuously casting metal strip over extended periods of time, it has been desirable to connect a replaceable nozzle to a separate crucible capable of holding a large quantity of molten metal. The resultant multipiece assembly has required sealing means to prevent leakage of molten metal between the component parts. A conventional nozzle assembly is representatively shown in U.S. Pat. No. 4,154,380 issued May 15, 1979 to W. Smith.
The sealing means of Smith is comprised of a tapered, frusta-conical surface on the casting nozzle which is adapted to mate with a corresponding tapered surface along the outlet passage from the crucible. The weight of the nozzle plus the metallostatic pressure head holds the mated, tapered surfaces together. These surfaces, however, required precise machining to obtain the required degree of sealing effectiveness. In addition, the closely mated surfaces do not allow for any differences in thermal expansion between the crucible and the nozzle body, particularly where the crucible and nozzle body are constructed from different refractory materials having different coefficients of thermal expansion. As a result, the nozzle can expand more than the crucible outlet passage opening, and very large lateral, side to side, forces developed between the crucible and nozzle have often caused the nozzle to fracture. If the nozzle should fracture severely, the flow of molten metal is no longer restricted by the relatively small nozzle extrusion orifice; large volumes of molten metal may then escape and damage the casting surface and any nearby auxiliary support equipment.
Thus, conventional nozzle assemblies, such as those taught by Smith, have lacked a mechanism for preventing nozzle fracture due to thermal expansion of the nozzle against the crucible and have lacked a safety device to prevent the sudden escape of large volumes of molten metal from a fractured nozzle.